Population structure has long been recognized as a catalyst for cooperation that cannot otherwise spread in a well-mixed society. And yet most theoretical analysis of this effect has assumed bi-directional social interactions, even though real-world interactions are often uni-directional. Directionality arises in real-life settings as the result of organizational hierarchy, social stratification, popularity effects, as well as endogenous mechanisms of network growth in online social networks. But the impact of directed interactions for cooperation have not been thoroughly studied, by either theoretical analysis or empirical experimentation. One reason why directionality may have been neglected is that, a priori, directional interactions would seem to only impede cooperation, because they remove the possibility of pairwise reciprocity [1]}, [2]}, [3]}.
